This invention relates to a system for purifying contaminated air.
A system for purifying contaminated air is disclosed in U.S. Pat. No. 4,890,581. In this system, the exhaust air from a tenter is fed as combustion air to the burner of a steam boiler. The burner is operated with a variable excess of air which depends on the load. In deviation from the usually desired lowest possible amount of excess air, the excess air is increased with decreasing boiler load. In this way it is possible in many cases, in spite of varying boiler loads, to purify all of the exhaust air from the tenter. Additionally this method reduces energy consumption and improves efficiency. Therefore, this system has been found to be very satisfactory. In principle it is also suitable for purifying the exhaust air from other sources than from tenters or singeing equipment.
However, in practical operation of systems constructed according to U.S. Pat. No. 4,890,581 it has been found that the actually desirable upper excess air limit of .lambda.=3.5 cannot be realized because the purifying effect is insufficient if the amount of excess air is very high. Therefore, in practical operation the upper excess air limit had to be kept at approximately .lambda.=2.5. In certain cases, where there is a greatly fluctuating boiler load or where the boiler is loaded only slightly, this is not sufficient.
In such cases it was therefore necessary to fall back to the prior art thermal afterburning process according to VDI [Association of German Engineers] Guideline 2442 of June, 1987. In this afterburning system, crude gas is heated by means of a surface burner to about 800.degree. C. and is then cooled again by means of a heat exchanger. The fuel supply is regulated in such a way that the temperature in the combustion chamber is kept constant at 800.degree. C. The combustion chamber is a non-insulated steel pipe around whose exterior the crude gas flows. The pipe thus acts as the last stage of the heat exchanger. In the heat exchanger the crude gas to be purified is heated up to almost 600.degree. C. so that fuel costs can be kept as low as possible. Such systems are expensive and have hardly any utility value except for reducing pollution. The exhaust heat still contained in the gas can rarely be used economically in an appropriate manner. Moreover, the gas-gas heat exchanger operated at a high temperature poses considerable material problems so that frequently a temperature higher than 800.degree. C. which would actually be desirable cannot be employed.
A surface burner for such afterburning systems is disclosed in DE-A-3,025,948. These burners are operated in that crude gas at a temperature up to almost 600.degree. C. flows around them and they are designed for a constant combustion temperature of about 800.degree. C. They are not suitable as conventional steam boilers operated with flame temperatures up to almost 1800.degree. C.
EP-A-0,212,245 discloses a system for burning halogen-ized hydrocarbons. The fire tube of a steam boiler is preceded by an internally insulated combustion chamber into which opens a special burner. The burner receives combustion air, fuel, the contaminants to be combusted as well as an atomizing fluid. The burner is operated at a constant flame temperature of about 1800.degree. C.